1. Field of the Invention
The invention relates to an arrangement which is intended for applying a tensioning force to a strapping band and is provided with a guide for the strapping band, with a transporting device which can move the strapping band, and with a drive for the transporting device.
2. Description of the Relevant Art
Various machines are already known for the purpose of applying a band or a plurality of bands by means of which an open or packed item is held together. Such machines serve to tension the band positioned around the item, to connect the band at the band ends, and to separate it from a supply roll. The operations of applying the band around the item which is to be strapped and of introducing the band, with the requisite overlapping of the band ends, into the machine may be carried out in a manual or automated manner in this case. However, in the case of machines which are provided with an arrangement of the generic type of the invention, the tensioning and subsequent connecting operations are usually carried out mechanically.
It is thus already known, for example, to route, and deflect, the band between at least two rubberized wheels, plastic wheels and/or belt drives, the band being tensioned with the aid of the resulting frictional forces. An arrangement of this type is described in Swiss Patent Specification 662 791. The disadvantage with this tensioning system, however, is that, as a result of the friction, the wheels and/or the belt drives wear relatively quickly, and the functional reliability is thus impaired. It cannot then be ensured that a specific desired tensioning force is actually applied to the band.
Other previously known tensioning systems which use a slotted drum for fixing a band end during the application of the necessary band tensioning are likewise unable to provide satisfactory results. In the case of such tensioning systems, it is necessary to fix in the drum a band section which is still connected to the band located on the supply drum. In particular when a high degree of band tensioning is to be applied, the band is subjected to considerable plastic deformation by buckling as a result of the slotted drum. It has been shown that such deformation has an adverse effect on functional capability, e.g. the tensioning force that can be applied.
Furthermore, it is also not possible in the case of such tensioning systems to compensate for the deviations from the desired tensioning force which occur as a result of disruptive influences. It is also disadvantageous that it is no longer possible for band sections deformed in this way to be transported reliably in guides of tensioning and closure heads. Since the deformed band section is the end of the respectively next strapping operation and still has to be transported through the entire guide, the only assistance which can be provided for this is to sever said band section prior to the next strapping cycle.
In the case of a further tensioning system which is already known, relative movement takes place between a tensioning lever and a so-called bell-shaped curve. A bell-shaped curve is to be understood as the contour of an end face of a rotatable cylinder on which one end of a tensioning lever is located. The tensioning-lever displacement caused by the rotary movement of the cylinder is used for tensioning the band. The disadvantage here, however, is that the path covered by the tensioning lever is predetermined by the bell-shaped curve and cannot be changed. Since the tensioning force applied to the band is dependent on a number of factors, for example the length, the material or the cross-section of the band, it is virtually impossible, given different use conditions, to apply a predetermined tensioning force to the respective band.